Bump Jepsen version to 0.2.3

[ligurio]

https://github.com/jepsen-io/jepsen/releases/tag/0.2.1

Useful changes for Tarantool tests:

• 0.2.1: nemesis.membership: an experimental namespace which supports writing membership-changing nemeses and generators. Users provide an implementation of the nemesis.membership.state/State protocol: a mostly-pure structure which defines how to observe the state of the cluster on a specific node, merging those node views, generating operations, applying those operations to the cluster, and (since clusters often resolve membership changes asynchronously) deciding when those operations have been completed. Given this object, the membership system handles spawning threads to observe the cluster state, evolves the given state machine towards a fixed state over time, and provides a stateful nemesis and generator that work together to perform membership changes. The resulting package can be combined with other faults through nemesis.combined.
• 0.2.1: Jepsen now logs the GIT hash and command line used at the start of each test, which makes it easier to reproduce results.
• 0.2.0 jepsen.cli now takes a --no-ssh option, which is helpful when running Jepsen against local systems, existing databases, or external APIs.
• 0.1.19 jepsen.generator.pure is basically stable for writing production tests at this point. See the namespace docs for details.

[Totktonada]

I would also note the following bugfix from 0.2.4:

• control.clj-ssh (and the new sshj backend) now include a concurrency-limiting semaphore, which prevents at least some (but not all) of the weird, nondeterministic bugs we've seen with session initiation.

jepsen-io/jepsen@30b2bd1